This invention relates to a semiconductor integrated circuit for enhancing a fail-safe function for operation stop of a vibrator-type oscillation circuit.
There is a fail-safe function of realizing safe performance as a strong demand for an LSI, etc., intended for vehicle-installed application. To realize the fail-safe function of an LSI, malfunction of the LSI is monitored using a watchdog timer, etc., regardless of the normal operation time or the stop time. Since a measure of accuracy is required for a clock supplied to the LSI, a main clock is obtained from a high-accuracy vibrator-type oscillation circuit, but it is necessary to take measures against stop of the vibrator-type oscillation circuit because of fluctuation of power supply voltage, etc.
As one of the measures, there is a technique of monitoring malfunction of LSI by a watchdog timer using an output clock of a ring oscillator always operating independently of the operation state of the vibrator-type oscillation circuit. FIG. 9 is a block diagram to show a configuration example of a conventional LSI for monitoring malfunction using a watchdog timer operating based on an output clock of a ring oscillator.
An LSI 10 shown in FIG. 9 includes a vibrator 51, a vibrator-type main oscillation circuit 11 for generating a main clock signal S11 based on an input-side oscillation capacity 53 and an output-side oscillation capacity 55, a function circuit block 13 containing a CPU, etc., for realizing the function intended by the LSI, a watchdog timer 15, and a ring oscillator 17 of low current consumption type always operating independently of the operation state of the main oscillation circuit 11.
The watchdog timer 15 uses an output clock signal S13 from the ring oscillator 17 and monitors the operation state of the function circuit block 13 operating according to the main clock signal S11. Thus, malfunction of the LSI 10 can be monitored independently of the operation state of the main oscillation circuit 11. The ring oscillator 17 used with the LSI 10 shown in FIG. 9 operates even at a low power supply voltage of about 1 V, is resistant to fluctuation of the power supply voltage, and consumes a small current.
Since a high-accuracy clock is required for a system LSI used with a digital clock, etc., a high-accuracy vibrator-type oscillation circuit supplies a main clock. In such a system LSI, it is necessary to guarantee that the oscillation circuit always normally operates and thus an oscillation stop detection circuit for directly monitoring output of a vibrator-type oscillation circuit is provided (for example, refer to Patent Literature 1). The oscillation stop detection circuit processes output of the oscillation circuit at a potential level, thereby detecting stop of the oscillation operation, controls power supply voltage of the oscillation circuit according to output of the oscillation circuit, and automatically restarts the oscillation circuit.    Patent Literature 1: JP-A-58-70630
As described above, the main clock supplied to the LSI is obtained from the high-accuracy vibrator-type oscillation circuit. However, when the vibrator-type oscillation circuit stops due to fluctuation of power supply voltage, etc., automatic operation recovery cannot be performed although an emergency step can be taken by monitoring malfunction of the LSI using the watchdog timer, etc. Thus, the function is insufficient as the fail-safe function. In the art shown in Patent Literature 1, output of the oscillation circuit is monitored at the potential level and when oscillation stops, the power supply voltage of the oscillation circuit is controlled and the oscillation circuit is restarted. Thus, intended operation is not guaranteed for fluctuation of the power supply voltage, etc.